1,3-disubstituted derivatives of oxazinan-2-ones are reportedly useful as inhibitors of 11-β-hydroxysteroid hydrogenase type 1 (“11-β-HSD1”) and for treatment of disorders associated with 11β-HSD1 activity including, for example, diabetes mellitus (e.g., type II diabetes), obesity, symptoms of metabolic syndrome, glucose intolerance, hyperglycemica (see, e.g., WO/2009/134400 and WO/2010/011314).
WO/2010/011314 describes several methods of making intermediates useful for making the 1,3-disubstituted oxazinan-2-ones. In one of the described methods, 1-chloro-3-phenylhex-5-en-3-ol is reacted with (S)-1-bromo-4-(1-isocyanatoethyl)benzene to form a mixture of diastereomers A and B as shown below.

However, the isolated yield of the desired diastereomer (B) was only 34%. Moreover, the preparation of (S)-1-bromo-4-(1-isocyanatoethyl)benzene uses triphosgene, a highly toxic reagent. Thus, it is desirable to have a more efficient process making diastereomer B, for example, by preparing diastereomer B in higher yield and/or a more optically pure form.